Arc lamp envelopes



Aug. 2

AMPLITUDE TIME FIGZD.

w. J. SOULE ETAL ARC LAMP ENVELOPES Filed July 25, 1968 FIG! FIGZA.

FIGZB FIGZC.

JNVENTOI? WILLIAM J. SOULE MARTIN L. DE POYII United States Patent US.Cl. 313-221 5 Claims ABSTRACT OF THE DISCLOSURE A metallic halideenvelope is provided for a high temperature alkaline vapor arc lampwhereby a reduction in the recombination time of dissociated moleculesin the arc plasma is produced. This reduction further provides animprovement in the modulation index of such lamps.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to are lamps and more particularly to protective envelopes forare discharge phenomena in high-temperature alkaline vapor arc lamps.

Description of the prior art The alkaline vapor arc discharge lamp hasbecome the primary laboratory source of far-infrared radiation. Suchlamps are now commercially available, filled with sodium, cesium,rubidium, and lithium to name but a few. It will be readily apparentthat some sort of protective envelope is required to contain the arcdischarge phenomena. These envelopes must maintain their structuralintegrity at elevated temperatures, and, on exposure to hot plasmaconstituents, have a coefficient of thermal expansion which iscompatible with that of the material to which the envelope must besealed and also must be highly transmissive of the infrared energy inthe desired wavelength bands.

Previously available alkaline vapor arc lamps have employed suchmaterials as quartz, sapphire and calcium aluminate as protectiveenvelopes. These lamps unfortunately have a relatively short lifetimedue to envelope fracture under conditions of lamp-induced thermal shock;i.e., the extremely high rate at which the envelope is heated when thelamp is started. In many applications, it is desirable to provide an arclamp which may be electronically modulated at relatively highfrequencies. An additional limitation of the prior art lamps is theirlow modulation index at pulse repetition frequencies above 100 to 1000pulses per second. This shortcoming is due to the relatively long timewhich is necessary for the molecules in the arc plasma to recombine whenthe lamp is turned oti and hence the long fall time of the recombinationradiation. Thus, when it is desired to electronically modulate the lampoutput at relatively high frequencies, the recombination radiationprecludes a depth of modulation much in excess of 80 percent.

OBJECTS OF THE INVENTION Accordingly, it is a primary object of thepresent invention to provide a new and novel protective envelope foralkaline vapor arc lamps.

It is another object of the present invention to provide an envelope ofthe above-described character which is suitable for high temperatureoperation.

It is an additional object of the present invention to provide anenvelope of the above-described character which enhances the lampmodulation index.

It is still a further object of the present invention to 3,525,896Patented Aug. 25, 1970 provide an envelope of the above-describedcharacter which is resistant to thermal shock.

SUMMARY OF THE INVENTION According to this invention, these objectivesare achieved by providing an arc lamp envelope fabricated of a metallichalide. These materials are particularly well adapted to use as an arclamp envelope due to their high melting temperature, resistance tostructural failure on exposure to hot alkaline plasma constituents,coeflicients of thermal expansion which are well matched to envelopeseal metals and excellent transmission of intrared energy to beyond ninemicrons. In operation, the arc plasma attacks the metallic halideenvelope to a limited extent thereby freeing a limited quantity of thehalogen. The halogen atoms act as a plasma dopant which decreases therecombination time of the dissociated molecules in the arc plasma by twoto three orders of magnitude. Thus the range of lamp modulationfrequencies is greatly widened and the lamp modulation index approachespercent.

These and other objects, features and advantages of the presentinvention will become more apparent from the detailed discussionconsidered in conjunction with the accompanying drawing. The scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectionalview of an arc lamp fabricated according to the present invention.

FIG. 2 is a graph illustrating the advantages of the present inventionwith respect to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIG. 1,there is shown a crosssectional view of a typical vapor arc lampstructure 10 in which the present invention may be embodied. Positiveand negative electrodes 12 and 14, respectively, between which there isan arc gap 16, are enclosed in an alkaline vapor filled envelope 18which is transparent in a desired wavelength region. The envelope issealed to end caps 20 and 22 which are electrically coupled to theelectrodes 12 and 14, respectively. The end caps 20 and 22 are of aconfiguration which facilitates the insertion of the lamp in a holder(not shown) which is coupled to an external power supply. It is to beunderstood that the lamp configuration illustrated is not in any way tobe construed in a limiting sense. The envelope could as well be, forexample, of a bulbous shape in the vicinity of the arc gap 16. Thepresent invention contemplates the use of a metallic halide as thematerial of which the lamp envelope 18 is fabricated.

The applicants have found that metallic halides, and in particular,magnesium fluoride or Irtran I which is a hot pressed polycrystallineform of magnesium fluoride manufactured by the Eastman Kodak Company ofRochester, N.Y., possesses the physical properties which are desirablein a high temperature arc lamp envelope. As established by themanufacturer, this material has a melting point of 1255 degreescentigrade and a coeflicient of thermal expansion of 11.0 10 over thetemperature range 25-200 degrees Centigrade. This coefficient is wellmatched to the coefficient of thermal expansion for annealed stainlesssteel which is 10.7 10" over approximately the same temperature range.Further, the thermal shock characteristics of Irtran I are particularlywell suited to the thermal environment of an arc lamp envelope.

The use of magnesium fluoride as an alkaline vapor arc lamp envelopeprovides a very significant improvement in the modulationcharacteristics of the lamp. It

has been found that the alkaline constituents in the arc plasma attackthe magnesium fluoride envelope material to a limited degree, thusfreeing minute quantities of fluorine. The fluorine atoms act as adopant in the arc plasma which results in a decrease in therecombination time of the dissociated molecules in the arc plasma. Bythis means, the recombination radiation fall time; i.e., the timerequired for substantially complete molecular recombination, is reducedby two to three orders of magnitude. The halogen atoms freed by thelimited alkali attack have an oxidation state of minus one and are thushighly reactive with the free alkali metal atoms of the arc plasma whichhave an oxidation state of plus one. When the power to the lamp is cutoff, the reaction between the halogen and the alkali occurs much morerapidly than when the alkali atoms recombine with oxygen, for example.The drastic reduction in the recombination time provided by the freehalogen atoms results in a dual benefit in the modulation characteristicof the arc lamp. First, for a given depth of modulation, the presentinvention permits the lamp to be operated at a higher pulse repetitionrate; i.e., more pulses per second. Second, for a given pulse repetitionfrequency, the present invention permits a greater depth of modulationto be achieved. The advantages of the present invention are illustratedin FIG. 2. Plot A illustrates the input energy amplitude applied to anarc lamp as a function of time. An ideal arc lamp would haveinstantaneous response and thus have an output which would corresponddirectly to the input energy. Plot B illustrates the performance ofprior art are lamps which is characterized by a fast rise to the peakamplitude I, a substantially flat-topped pulse during the application ofthe input energy, and a slowly decaying output resulting from therecombination radiation. It will be noted that for pulse repetitionfrequencies on the order of 100 pulses per second or more, therecombination radiation does not have suflicient time to decay beforethe start of the next pulse. Since the pulses overlap, the depth ofmodulation which is achievable generally does not exceed 80 percent.Plot C of FIG. 2 illustrates that through the use of the presentinvention the recombination time is substantially reduced thus resultingin a much improved depth of modulation which approaches. 100 percent.Plot D further illustrates that if the depth of modulation availablewith the prior art lamp of Plot B is satisfactory in a givenapplication, the number of pulses which may be transmitted in a unittime is significantly increased. Here, there is shown an improvement ofthree and one-half times in the available pulse repetition rate. Inactual practice, an improvement by a factor of about may be provided.Although magnesium fluoride has been described in the foregoing discussion for the purposes of illustration, it is to be understood thatthe halides of lithium, sodium, magnesium, calcium, barium, aluminum,zirconium, and yttrium are also suitable for use in the practice of thepresent invention.

The extent of the attack of the metallic halide envelope by the alkalineconstituents of the arc plasma is not precisely known. It is known,however, that the amount of halogen necessary to achieve the reductionin recombination time is very minute and that the phenomenon occurs tosuch a limited extent as to have no material effect on the lamplifetime. This is to say that the total power output of the lamp overits lifetime, the number of pulses times the peak pulse power, is notnoticeably reduced.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efliciently attained, and,since certain changes may be made in the above construction withoutdeparting f5om the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention thereindescribed, and all statements of the scope of the invention, which, as amatter of language, might be said to fall therebetween.

Having described what is new and novel and desired to secure by LettersPatent, What is claimed is:

' 1. An alkaline vapor arc lamp comprising an envelope containing analkaline vapor, means for sealing each end of said envelope, a pluralityof electrodes sealed Within said envelope and in electrical contact withsaid sealing means, and

said envelope being formed of a material which releases a halogen whenattacked by said alkaline vapor during operation of said lamp.

2. Apparatus as recited in claim 1 where said envelope is formed of ahalide of a metal selected from the group consisting of lithium, sodium,magnesium, calcium, barium, aluminum, zirconium, and yttrium.

3. Apparatus as recited in claim 2 wherein said halide is a fluoride.

4. Apparatus as recited'in claim 2 wherein said envelope is formed ofmagnesium fluoride.

5. Apparatus as recited in claim 4 wherein said magnesium fluoride is ina hot-pressed polycrystalline form.

References Cited UNITED STATES PATENTS RAYMOND F. HOSSFELD, PrimaryExaminer US. Cl. X.R. 2202.l; 313-412

